The invention concerns a method of nuclear magnetic resonance (NMR) spectroscopy or tomography, wherein a sequence of radio frequency (RF) pulses consisting of an excitation pulse followed by a sequence of in general equidistant refocusing pulses is applied to a spin ensemble like in a CPMG multi echo experiment, and wherein the magnetization generated after an initial excitation pulse is transferred to or close to the static pseudo steady state of some refocusing flip angle α1 and kept in or close to the static pseudo steady state of any ensuing refocusing pulse αn.
The basic spin echo refocusing experiment is known from Hahn E L, Spin Echoes, Phys. Rev. 80:580-594 (1950) (=Reference [1]).
Nuclear resonance signals are often measured through the spin echo method [1]. Therein, the excited magnetization is inverted after a period te/2 through a refocusing pulse, and after a further period te/2, a spin echo is formed. At the time of the spin echo, effects acting on the spins such as chemical shift, susceptibility, field inhomogeneity are refocused such that all spins have a coherent signal phase with respect to these effects.
The signal maximum is obtained when the flip angle of the refocusing pulse is exactly 180°. Such an ideal flip angle can only be approximated in practice such that in particular for methods which are based on formation of many spin echoes, signal losses result on the basis of the deviation of the flip angle of the refocusing pulses of 180°. Such a deviation can occur either through technical facts or on purpose, e.g. In applications on human beings to stay within the allowed values for radio frequency exposure measured by the SAR (=specific absorption rate).
Literature suggested a series of measures for limiting the corresponding signal losses. This is on the one hand the so-called Carr-Purrcell-Meiboom-Gill method [2] wherein the refocusing pulses are partially automatically compensated for through corresponding signal phase shift between excitation and refocusing pulses.
It was shown that with such a sequence and long echo trains, high echo amplitudes can be obtained even with small refocusing flip angles [3].
The use of one [4] or more [5,6] different optimized flip angles over the first refocusing periods of the multi echo train further increases the echo amplitude.
All of these methods are based on the fact that magnetization is transferred into a so-called pseudo steady state through application of a sequence of equidistance pulses P(αφ) with identical amplitude α and phase φ where it could be shown [7] that for any α,φ, an infinite variety of such pseudo steady states exists. The term pseudo steady state originates from the fact that a constant signal amplitude can be achieved only when relaxation effects are neglected.
The modifications described in references [4]-[6] therefore have the aim to bring magnetization as close as possible to the one particular pseudo steady state with maximum amplitude, which is called the static pseudo steady state, which in the following is designated as PSS0. In addition to maximation of the signal intensity, the signal modulations which occur during use of continuously constant flip angles, are eliminated.
The common feature of all these methods is therefore to transfer the purely coherent magnetization M(x,y), generated by the initial excitation pulse, as continuously as possible to or at least close to PSS0 and keep PSS0 through further application of pulses with constant α,φ.